Refactor WebpageRequest validation and tests

- Refactor spec examples for WebpageRequest to use shoulda matchers.
- Modify Cucumber WebpageRequests step definitions to use FactoryGirl.
- Validate presence of associated User on WebpageRequest model.
- Only validate if a URL is reachable if it is the correct format.
- Don't call the user_url method for generating the FriendlyId slug
  if the url is blank.
- Use domain name for VCR cassette name as opposed to example
  description.
- Include FactoryGirl::Syntax::Methods in order to use FactoryGirl
  methods without calling FactoryGirl directly.
- Update VCR cassettes.